Comparison between OpenFOAM CFD & BEM theory for variable speed – variable pitch HAWT

Abstract

OpenFoam is used to compare computational fluid dynamics (CFD) with blade element momentum theory (BEM) for a variable speed – variable pitch HAWT (Horizontal Axis Wind Turbine). The wind turbine is first designed using the BEM to determine the blade chord, twist and operating conditions. The wind turbine blade has an outer diameter of 14 m, uses a NACA 63–415 profile for the entire blade and root to tip twist distribution of $15^\circ$ (Figure 3). The RPM varies from 20–75 for freestream velocities varying between 3–10.5 m/s (variable speed) and a constant RPM of 78.78 for velocities ranging between 11–25 m/s (variable pitch). OpenFOAMis used to investigate the wind turbine performance at several operating points including cut-in wind speed (3 m/s), rated wind speed (10.5 m/s) and in the variable pitch zone. Simulation results show that in the variable-speed operating range, both CFD and BEM compare reasonably well. This agreement can be attributed to the fact that the complex three-dimensional flow around the turbine blades can be split into two radial segments. For radii less than the mid-span, the flow is three-dimensional, whereas for radii greater than the mid-span, the flow is approximately two-dimensional. Since the majority of the power is produced from sections beyond the mid-span, the agreement between CFD and BEM is reasonable. For the variable-pitch operating range the CFD results and BEM deviate considerably. In this case the majority of the power is produced from the inner sections in which the flow is three-dimensional and can no longer be predicted by the BEM. The results show that differences in pitch angles up to $10^\circ$ can result to regulate the power for high wind speeds in the variable-pitch operation zone